Diethylbenzenes have three isomers namely ortho-, meta- and para-. The para isomer is industrially more important than the other two isomers. It is a high value chemical having immense industrial importance by virtue of its utility as a desorbent in the selective recovery of para-xylene from isomeric C8 raffinate, by well known “PAREX” process innovated by M/s Universal Oil Product (USA). The demand for para-diethylbenzene is bound to increase in the coming years with the continuous growth in para-xylene demand.
Diethylbenzenes can be conveniently synthesized by using existing alkylation catalysts like AlCl3, HF, BF3 etc. However, conventional catalyst is not selective to the para-isomer, Isomers of aforesaid ortho-, meta- and para-diethylbenzene can result in equilibrium concentration according to thermodynamics. Thermodynamic equilibrium composition of the three isomers at about 425° C. is as follows: ortho-diethyl benzene:meta-diethyl benzene:para-diethyl benzene=19:54:27. These isomers have very close boiling points to each other and the relative volatility is nearly one. Hence separation is difficult and is quite expensive. In addition, with AlCl3 type catalyst, not only is separation difficult, it is impossible to avoid loss of raw material due to multiple alkylation giving polyalkylated heavy products. Moreover, due to strong acidity, disposal of catalyst causes serious environmental pollution apart from corrosion of equipments during operation of the process. Another approach for producing para-diethyl benzene is through adsorptive separation of para-isomer from mixed diethyl benzene isomers, which are produced during ethylbenzene/styrene manufacture.
Solid acid catalysts, particularly zeolites have been known to be very useful to replace the earlier AlCl3, HF, BF3 type catalysts. A new type of zeolite catalyst, which is known as ZSM-5, was discovered by Mobil Oil Corporation in 1972, has been used for many petrochemicals and hydrocarbon processes. The particulars of the method of production of the catalyst are disclosed in U.S. Pat. No. 3,702,886 and the details of the alkylation processes are revealed in an article published in the “Oil and Gas Journal, Sep. 26, 1977 by P. J. Lewis. The pores of this sort of zeolites have a uniform aperture. Therefore, hydrocarbons smaller than the pore dimensions are adsorbed and larger hydrocarbons are rejected. Hence it is frequently referred to as a “molecular sieve”. These materials are known to possess “reactant selectivity”, “product selectivity” and “transition selectivity”. Details about the different kind of selectivity as exhibited by these materials are described in the book “Shape Selective Catalysis in Industrial Applications” Second Edition, by N. Y. Chen, W. E. (Garwood, F. G. Dwyer, Marcel Dekker, Inc., 1996. There are many precedents in industry making use of these characteristics to conduct chemical reaction, but usually in a given process only one kind of selectivity is achieved at a time, viz., either reactant selectivity, or product selectivity or transition state selectivity. The ZSM-5 zeolite catalyst is characterized by its selectivity, being able to satisfy the needs for high selectivity to products of different molecules, but it still falls short of expectation in respect of isomers of same kind of product. For instance, when toluene is alkylated with methanol over ZSM-5 zeolite catalyst, selectivity for xylenes are very high, but the ratio of isomers of xylenes namely ortho, meta- and para-xylene remains near thermodynamic equilibrium compositions. The details are reported in J. of Catalysis. Vol 67, page 159, 1981 by W. W. Kaeding et.al.
Various techniques to enhance shape selectivity of medium pore aluminosilicates have been reported. U.S. Pat. Nos. 4,086,287; 4,094,921 and 4,117,024 describe catalytic processes for selective ethylation of monoalkylbenzene (toluene, ethylbenzene) to produce para ethyltoluene, para-diethylbenzene, using crystalline aluminosilicate zeolite ZSM-5 modified with oxides of phosphorus, antimony, boron, magnesium and/or steaming and coking. U.S. Pat. Nos. 4,117,026 and 4,128,592 describe catalytic processes for the selective production of para dialkyl substituted benzenes contain alkyl group having 1 to 4 carbon atoms using aluminosilicate zeolite modified with difficultly reducible oxides and further modified by coking. Similar processes have also been described in U.S. Pat. 4,143,084. Catalysts and processes for selective production of para-dialkyl substituted benzenes have been described in U.S. Pat. Nos. 4,379,761 and 4,465,886. The catalyst as described in U.S. Pat. No. 4,379,761 comprises a porous crystalline aluminosilicate zeolite ZSM-5 having deposited silica thereon and having incorporated there in phosphorus. U.S. Pat. No. 4,465,886 describes the selective conversion of certain hydrocarbon feed stock to product rich in para-dialkyl substituted benzene over a catalyst composite comprising of crystalline aluminosilicate zeolite ZSM-5 having deposited there on a coating of silica which extensively covers and resides substantially exclusively on the external surface there of. U.S. Pat. No. 4,465,886 teaches that the charge stock used for the selective production of para dialkyl substituted benzene includes a hydrocarbon precursor selected from the group consisting of mono-alkyl substituted benzene having 1-4 carbon atoms in the alkyl substitutent, such as toluene, ethylbenzene, propylbenzene or butylbenzene and a mixture of such precursor or benzene with an alkylating agent containing from 1to 4 carbon atoms, (see col.10 lines 3-12). It also teaches that ‘The use of mixed aromatics as feed is also possible’. For example a mixture of ethylbenzene and toluene is converted selectively to a mixture rich in para-dialkyl substituted benzene such as para diethylbenzene and para ethyltoluene, the latter predominating at high toluene to ethylbenzene ratios in the feed” (see column 10, lines 22-27). Similar teaching are also described in U.S. Pat. No. 4,421,941 col. 12, lines 40-49; col.12. lines 59-64 and in U.S. Pat. No. 4,477,583 vol. 10. lines 3-12, and col.10, lines 22-27
Thus the aforementioned art describes simultaneous alkylation of a mixed aromatics feed to produce individual para-disubstituted product depending on the components in the feed mixture. Based on the above teaching, it is completely non-obvious to consider a feed mixture wherein a particular component of the mixture is selectively alkylated, while other components remains largely unaffected. For example, consideration of a C8 aromatics stream contain primarily ethylbenzene and xylenes, wherein ethylbenzene is disproportionate or alkylated over a selectivated metallosilicate catalyst, to produce para-diethylbenzene while the xylene isomers in the mixture remain substantially unaffected.
U.S. Pat. No. 4,613,717 describes process for producing 1,4-dialkylbenzene with high yield and selectivity using silicic acid ester modified AZ-type zeolite such as Al-AZ, B-AZ, Cr-AZ. The-process is described for conversion of monoalkyl benzene having an alkyl group containing 1-3 carbon atoms. U.S. Pat. No. 5,233,111 describes catalytic process for selective alkylation of aromatic hydrocarbons, with special reference to 1,1-biphenyl, diphenyl ether, naphthalene etc over de-aluminated mordenite zeolite. The patent also describes propylation of ethylbenzene using mordenite based catalyst.
Disproportionation of toluene (U.S. Pat. No. 5,367,099) and disproportionation of ethylbenzene (U.S. Pat. No. 5,382,737) is described over ex-situ selectivated zeolite catalyst. A selectivated and steamed zeolite is contacted with toluene or ethylbenzene accomplishing high para-selectivity of the product para-xylene or para-diethylbenzene under conversion conditions of temperature from ˜100° C. to ˜700° C., pressure from about 0.1 atmospheres to 200 atmospheres, and a WHSV (weight hourly space velocity) of from about 0.08 to about 200 h−1, and a hydrogen to hydrocarbon mole ratio of from 0 to 100.
It is to be emphasised that any product must meet certain specification for the purpose of commercial application. In other words, the impurity profile of a product is equally important as that of purity of the product. In case of para-diethylbenzene, some of such specification as laid down for the purpose of application as desorbent in PAREX process of M/s UOP are Purity 95 wt % (min), C8 aromatics <0.2 wt %, C9 aromatics <0.4 wt %, C10+heavy aromatics (other than diethyl benzene isomers), <0.5%, Nitrogen 1 ppm (max), Sulphur 1 ppm (max), Chloride 1 ppm (max), Carbonyl Number 1 ppm by weight (max), Bromine Index 20 (max),
The trace amount of unsaturated hydrocarbons in aromatic hydrocarbons are measured by potentiometric titration of the sample with potassium bromide-potasium bromate solution and is reported as ‘Bromine Index’ of the sample in consideration. The ‘Bromine Index’ of a hydrocarbon is defined as the number of milligrams of bromine consumed by 100 grams of the hydrocarbon sample under given conditions. ‘Bromine Index’ is usually determined by following the American Standard Test Method No. D-1491, (D1491-78). The sample is dissolved in a special solvent (comprising glacial acetic acid, carbon tetrachloride, methanol, sulphuric acid and a solution of mercuric chloride in methanol), and is titrated with 0.02 N potassium bromide-potassium bromate mixture. End point is determined potentiometrically using a glass-platinum electrode pair.
The trace amount of carbonyl function (in-the range of 0.1 to 100 milligram per litre) present as ketone or aldehyde in C5 to C18 hydrocarbons or alcohols are measured by chemical analysis and are reported as ‘Carbonyl Number’ of the sample under consideration. The ‘Carbonyl Number’ (also called Carbonyl No), is defined as milligrams of carbonyl functional per litre of the sample using acetophenone as standard. ‘Carbonyl Number’ of ‘Parex’ desorbent (para-di-ethylbenzene) is determined by following the American Standard Test Method No E-411 and UOP Method No 624. The results are normally used as indication of oxygen exposure of the ‘Parex’ and ‘Molex’ feedstocks. The ketone and aldehyde (carbonyls) in the sample is extracted and reacted with an acidic, alcoholic 2,4-dinitrophenyl hydrazine to form a phenyl hydrazone. Alcoholic potassium hydroxide is added to stop the reaction and convert the yellow hydrazone to a pink compound, the intensity of which is proportional to the carbonyl concentration. The intensity of the colour is measured at 480 nm in a spectrophotometer and the carbonyl content of the sample is determined from a standard calibration awe from which the carbonyl number is determined.
U.S. Pat. No. 5,811,613 issued on Sep. 22, 1998 and assigned to the assignee herein describes an effective process for production of para-diethylbenzene using ethylbenzene, ethanol, and steam in the presence of a pore size controlled galloaluminosilicate zeolite catalyst. However, in commercial operation of this patent, it was observed that the final para-diethylbenzene product, recovered from reactor effluent (by separation of the reactant and heavier hydrocarbons by-products), contains some objectionable impurities, like unsaturated hydrocarbons, and carbonyl compounds. For example, the final para-diethylbenzene obtained from the operation of the process described in the said U.S. Pat. No. 5,811,613, has been found to have ‘Bromine Index’ in the range of 60-70 and ‘Carbonyl Number’ in the range of 30-40. Such product needs further purification steps like treatment with acids to get rid of unsaturated hydrocarbons, and treatment with suitable organic reducing agents to reduce the carbonyl compounds, in order to meet the specification of the product for commercial applications Such steps not only add to the cost of production of the product, but also cause a loss of the valuable finished product. In addition, in some cases it may produce waste streams which may not be environment friendly.
Enhancement of para-selectivity, (the fraction of para-isomer in a mixture of disubstituted aromatics), by treatment with organosilicon compound is usually referred to in the art as ‘selectivation’ by ‘silanation’. The organosilicon compound is usually known as ‘selectivating agent’. The method normally comprises contacting zeolite with organosilicon compound, separation/removal of solvent (if used), and calcination of zeolite to deposit silica or polymeric silica as a layer on the zeolite.
It is known in the art that the efficiency of silica deposition in order to enhance the selectivity of the zeolite depends on the nature or the kind or the type or the molecular structure of the selectivating agent, i.e. the organosilicon compound employed. The efficiency of silica deposition also depends on the temperature of silanation, the solvents or the carrier for the organosilicon compound, the method or procedure adopted for the selectivation. Pretreatment of the zeolite, i.e. treatment carried out before selectivating the zeolite has also been found to affect the final selectivity of the zeolite. Also post-treatment, i.e. treatment after selectivating the zeolite have also been described in the art to further improve the selectivity of the zeolite for particular hydrocarbon conversion processes.
Selectivation of zeolites by silanation can be carried out in vapour phase or liquid phase. Liquid phase silanation is also referred as ‘ex-situ silanation’, or ‘ex-situ selectivation’. The zeolite is impregnated with an organosilicon compound dissolved or dispersed in a carrier or solvent followed by calcination of such treated zeolite in an oxygen containing atmosphere under conditions sufficient to remove organic material therefrom and deposit siliceous material on the zeolite. Such ex-situ silanation may result in deposition of at least 1% by weight of siliceous material on the catalyst or zeolite.
Examples of various patents, which teach ex-situ selectivation of zeolites to enhance para-selectivity are U.S. Pat. No. 3,698,157 (to Allen et. al.), U.S. Pat. No. 4,002,697 (to Chen), U.S. Pat. Nos. 4,127,616 and 4,402,867 (both to Rodewald). U.S. Pat. No. 3,698,157 (to Allen et al) describes improved chromatographic separation of C8 aromatic mixture for the recovery of para-xylene therefrom using aluminosilicate zeolite H-ZSM-5 modified with octadecyltrichlorosilane. U.S. Pat. No. 4,002,697 (to Chen) describes preparation of catalyst for xylene manufacture by toluene methylation. Silica modified catalysts employed for the purpose were based on zeolites like ZSM-5, ZSM-11 or ZSM-21 of average crystal size of greater than 0.5. μ, having surface deactivated by reaction with compounds of nitrogen or silicon, i.e. phenyl carbazole or dimethyldichlorosilane, (which are sufficiently large as to be unable to penetrate pores of crystalline aluminosilicate) followed by calcination Pyridine was employed as a solvent for dimethyldichlorosilane. U.S. Pat. No. 4,127,616 (to Rodewald) describes catalysts suitable for alkylation of toluene with methanol or ethanol, and toluene disproportionation to obtain selectively the corresponding para-dialkyl benzene. The catalyst was prepared by deposition of large organosilicon compound e.g. polymeric phenylmethyl silicone or polymeric methylhydrogen silicone on crystalline aluminosilicate H-ZSM-5, followed by calcination. Silica modified zeolite catalysts have been described in U.S. Pat. No. 4,402,867 (to Rodewald), utilizing aqueous emulsion of methylhydrogen silicone. Such catalysts contain added amorphous silica within the interior of crystalline structure of zeolite. The organosilicon compound employed here is small enough to enter pores of the zeolite.
When ex-situ selectivation process is repeated more than once, the procedure is referred to as ‘multiple selectivation’ or ‘multiple silanation’. In multiple selectivation, the zeolite is treated at least twice, generally from 2-6 times with a liquid medium containing the organosilicon compound(s). In multiple selectivation method, the zeolite is calcined after each impregnation of organosilicon compound. Examples of multiple silanation are found in U.S. Pat. No. 4,060,568 (to Rodewald), U.S. Pat. Nos. 4,283,306 and 4,449,989 (both to Herkes), U.S. Pat. No. 5,349,114. (to Lago et al), U.S. Pat. No. 5,495,059 (to Beck et al), U.S. Pat. No. 5,552,357 (to Lago et al), U.S. Pat. No. 5,574,199 (to Beck et al.), U.S. Pat. Nos. 5,726,114 and 5,990,365 (to Chang et al).
Modification of zeolites described in U.S. Pat. No. 4,060,568 (to Rodewald), comprises preparing crystalline aluminosilicate zeolite catalyst containing amorphous silica within the interior crystalline structure of ZSM-5, by exposing zeolite to volatile silane of small molecular dimension, which preferably enters the pores of zeolites, followed by treatment with aqueous ammonia and calcination The patent describes a catalyst modified by three such treatments with intermediate calcination after each treatment, but provides no description of any enhancement in catalytic selectivity or activity over that which might follow from a single such treatment.
U.S. Pat. No. 4,283,306 and U.S. Pat. No. 4,449,989 (both to Herkes) also describe methods of modifying crystalline silica catalyst by application of such silica sources as tetraethylorthosilicate (TEOS), or phenyl methyl silicone. Interestingly, performance of the catalyst treated once with a TEOS solution followed by calcination, was better than that of )catalyst treated twice with TEOS, and calcined after each treatment, thus showing that twice treated catalyst is less active and less selective than once treated catalyst as measured by methylation of toluene by methanol. This indicates that multiple ex-situ silanation confers no advantage over single silanation, rather results in adverse effect on para-dialkylbenzene selectivity. U.S. Pat. No. 5,349,113 (to Chang et al) describes modification of molecular sieve catalyst by treating with substantially aqueous solution of a water soluble organosilicon compound. The method includes concurrent preselectivation and activation to get activated catalyst. The invention also comprises in-situ selectivation by passing a high efficiency para-xylene selectivating agent along with the reactants. U.S. Pat. No. 5,349,114 (to Lago et al) describes shape-selective hydrocarbon conversion over modified catalytic molecular sieve, which has been modified by (i) being preselectivated with a first silicon containing compound and (ii) subsequently steamed at about 280° C. to 400° C. The, patent indicates that the molecular sieve is modified in as-synthesized conditions. U.S. Pat. No. 5,495,059 (to Beck et al) also describes multiple ex-situ selectivation sequence employing an aqueous carrier for the organosilane compound. Each sequence includes an impregnation of the molecular sieve with the selectivating agent and a subsequent calcination of the impregnated molecular sieve. Selectivation of molecular sieves has been described during extrusion by agglomerating with organosilicon compound by Chang et al in U.S. Pat. No. 5,541,146. U.S. Pat. No. 5,552,357 (to Lago et al) describes catalyst modification by treatment of ZSM-5 in as-synthesised or in ion-exchanged form, first by treatment with a silicon containing polymer (propylamine silane polymer) in substantially aqueous solution, followed by calcination The catalyst was further in-situ selectivated with a second silicon containing compound. For multiple ex-situ selectivation during first stage, i.e. during treatment with propylamine silane polymer, the catalyst was calcined after first treatment and before the second treatment.
Post-treatment of selectivated zeolite with a dealuminizing agent, e.g. monovalent or polyvalent acids, triethylene diamine, urea, ethylenediamine tetra acetic acid, ammonium hexafluorosilicate is described in U.S. Pat. No. 5,567,666 (to Beck et al). U.S. Pat. No. 5,574,199 (to Beck et al) describes shape-selective aromatization with a catalytic molecular sieve, which has been modified by multiple ex-situ selectivation method. The method involves exposing the catalytic molecular sieve to at least two selectivation sequences, each sequence comprising contacting the catalyst with dimethylphenylmethyl polysiloxane in a solvent, followed by calcination. U.S. Pat. No. 5,726,114 (to Chang et al.) describes a method for modifying intermediate pore catalytic molecular sieve by multiple ex-situ selectivation process by contacting the zeolite with an aqueous emulsion comprising of a silicon -containing selectivating agent stabilized with the aid of surfactant and calcining the contacted molecular sieve after each impregnation of silica. The method further comprises of mild steaming of the silica deposited zeolite and also in-situ trim selectivation of the ex-situ selectivated zeolite. U.S. Pat. No. 5,990,365 describes a method for preparation of a catalyst comprising ZSM-5, rhenium and a selectivating agent, e.g. either coke or siliceous material or a combination thereof. The multiple selectivation is carried out by (i) combining a bound form of zeolite with an organosilicon compound (ii) calcining the organosilicon containing material to remove organic material therefrom to deposit siliceous material on the bound ZSM-5 and (iii) repeating steps (i) and (ii) at least once.
While the above mentioned art is of interest, there is no suggestion of enhancing the selectivity of metallosilicate by treatment with aqueous water after the zeolite has been contacted with organosilicon compound and before calcination of the zeolite to improve the silanation efficiency. There is also no suggestion in any of the prior art known to the applicants, of multiple silanation of metallosilicates without any intermediate calcination of organosilicon compound treated zeolite after each silanation. Additionally, there is no suggestion of recycling the solvents/carriers for multiple silanation.
In addition, the aforementioned art has always been directed towards improvement of para-isomer selectivity of the products. There has been no suggestion for providing a catalyst composite possessing concurrently reactant, product and transition state selectivity for a given process.
Therefore, it would be a significant advance and improvement in the art to overcome the difficulties, disadvantages and deficiencies associated with conventional methods and procedures for modifying catalytic metallosilicates, molecular sieves modified by such methods and the process of shape selective hydrocarbon conversion using such modified catalytic molecular sieves